Available-for-Sale Debt Securities and Short-Term Investments

No definition available.

Name:	ago_AvailableforSaleDebtSecuritiesandShortTermInvestments
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Period Type:	instant

- Definition

Represents the number of fixed maturity securities and short-term investments which have been valued using model processes set by the entity.

+ References

No definition available.

+ Details

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Balance Type:	na
Period Type:	instant

- Definition

Represents the number of sources on which credit spread is based.

+ References

No definition available.

+ Details

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Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

This element represents the fair value investments categorized as other which are not in and of themselves material enough to require separate disclosure.

+ References

No definition available.

+ Details

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Namespace Prefix:	ago_
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Balance Type:	debit
Period Type:	instant

- Definition

Represents the percentage of CDS contracts which are fair valued using minimum premium. Minimum premium is the minimum rate that the company would currently charge to assume similar risks.

+ References

No definition available.

+ Details

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Namespace Prefix:	ago_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Estimated rate of prepayment of principal on financial assets for initial fair value measurement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=109247956&loc=d3e107207-111719

+ Details

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- Definition

For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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-Topic 275
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+ Details

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Balance Type:	na
Period Type:	duration

- Definition

Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 45
-Paragraph 1A
-URI http://asc.fasb.org/extlink&oid=99377092&loc=SL75136599-209740

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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Fair Value Measurement - Narrative (Details) $ in Millions	9 Months Ended	12 Months Ended
Fair Value Measurement - Narrative (Details) $ in Millions	Sep. 30, 2018 USD ($) source Security	Dec. 31, 2017 USD ($)	Jun. 30, 2018
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Percentage of CDS contracts which are fair valued using minimum premium	0.20%	16.00%	0.20%
Recurring [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Other invested assets	$ 44	$ 48
Recurring [Member] \| Level 3 [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Number of fixed maturity securities valued using model processes \| Security	119
Fixed maturity securities	$ 1,411
Equity Securities, FV-NI	$ 2	$ 2
Available-for-Sale Debt Securities and Short Term Investments [Member] \| Recurring [Member] \| Level 3 [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Percentage of fixed maturity securities valued using model processes to the Company's fixed-income securities and short-term investments at fair value	13.00%
Total [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Number of sources of credit spread \| source	3
Measurement Input, Default Rate [Member] \| Total [Member] \| Recurring [Member] \| Level 3 [Member] \| Minimum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Assumption, Date of Securitization or Asset-backed Financing Arrangement, Transferor's Continuing Involvement, Servicing Assets or Liabilities, Prepayment Speed	2.44%	1.72%
Measurement Input, Default Rate [Member] \| Total [Member] \| Recurring [Member] \| Level 3 [Member] \| Maximum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Assumption, Date of Securitization or Asset-backed Financing Arrangement, Transferor's Continuing Involvement, Servicing Assets or Liabilities, Prepayment Speed	3.17%	2.55%